When producing gravure cylinders, the image originals are transferred to a copper cylinder by conventional methods. Small wells, which are separated by lands, are produced on the copper cylinder either by engraving or by etching. If a part of an image is to appear dark, the cylinder has a very large number of large and deep wells in this area. If a part of the image is to appear light, the wells are correspondingly small and shallow.
In practice, the transfer of the image impression to the cylinder, in the form of small and large wells is never so accurate that, upon printing, the resultant image actually corresponds to the desired original. It is, therefore, necessary to subsequently enlarge or reduce the size of these wells by an appropriate amount. If a color at one location appars too light, the wells in this region of the cylinder must be so enlarged that more printing ink is taken up by the well and can be transferred to the image.
In this correction process, the lands are protected by a so-called hard etching ink. This hard etching ink is a black paste which is rolled, using a roller, in a very thin layer onto the lands in the image area to be corrected. When the cylinder surface consists of copper, the copper will be dissolved in the well upon etching with an iron-III salt solution, and, as a result, the well will be enlarged.
However, gravure cylinders for large printing runs must be chromed. After chroming, a sample print must then be run with the chromed cylinder. If it is found during this run that the depth of color in some image areas appears too light, the hard etching ink is rolled onto the lands of the chromed cylinder in those particular image areas, in the manner described above; image areas which are not to be corrected are masked with an asphalt varnish. After these protective measures, the cylinder is treated with an etching solution which dissolves the chromium coating in a controllable manner, so that the volume of the wells is enlarged.
In this etching process, it is very important that the etching solution uniformly wets the part of the image to be corrected. If only partial wetting takes place, this image area will be unevenly etched and spots and streaks will appear in the finally printed image. The demand made of a good etching medium is, therefore, that it uniformly wet the zones to be corrected and start the etching process at a uniform rate. It is also extremely important that the etching medium not attack the hard etching ink protecting the lands. Otherwise, etching of the lands will also start and, as a result, during the printing process there will be an overflow and damage to the cylinder.
A disadvantageous characteristic of the currently commercially available etching solutions for chromium coatings, and which contain zinc chloride as the active substance, is that they effect relatively poor wetting of the lands which have been rolled and protected with hard etching ink. This is particularly pronounced in image areas which contain very small wells and very broad lands. In these areas, a very high proportion of the surface is covered with hard etching ink, which is water-repellent. However, it is precisely these areas which must be wetted particularly well by the etching solution if the formation of spots and streaks in the printed image is to be avoided. On the other hand, however, it must be ensured that the etching medium does not attack the lands protected with hard etching ink. Specifically, if the hard etching ink is wetted well, there is, at the same time, a danger that the chromium coating of the lands located beneath the ink will also be attacked. Thus, apparently contradictory demands are made of the etching solutions.
A further disadvantage of the etching media used hitherto is due to the zinc chloride contained therein. This is because, for reasons of environmental protection, the zinc must be removed as completely as possible from the exhausted etching medium before the latter passes into the effluent. For this purpose, the exhausted etching medium solution is usually treated with sodium hydroxide solution, which results in the formation of sparingly soluble zinc hydroxide. However, it has been found in practice that accurate metering of the sodium hydroxide solution presents very great difficulties. If too small an amount of sodium hydroxide solution is metered into the exhausted etching solution containing zinc ions, too much zinc will remain in solution. If, on the other hand, too large an amount of sodium hydroxide solution is metered in, the zinc hydroxide already precipitated redissolves to produce the zincate, which thus, passes into the effluent. For this reason, it has not proved possible in practice to reduce the zinc content to below the maximum value of about 2 mg of zinc per liter, demanded for effluents.